1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly, to a color cathode ray tube which can increase impact resistance by having an optimum curvature coefficient of a shadow mask.
2. Discussion of the Related Art
Generally, color cathode ray tubes are the most commonly used display devices, in which an electron beam emitted from an electron gun hits a fluorescent film in a vacuum state of high temperature to display images.
FIG. 1 is a side view showing an inside of a color cathode ray tube according to a related art, and FIG. 2 is a perspective view showing a shadow mask according to a related art.
Referring to FIG. 1, the cathode ray tube includes: a panel 1 having a fluorescent surface 1a and a face 1b; a shadow mask 3 for selecting a color of an electron beam emitted from an inside of the panel 1; a frame 4 for fixing the shadow mask 3; a stud pin 5 for fixing the frame 4 to the panel 1; a spring 6 for connecting the stud pin 5 to the frame 4; a funnel 7 engaged to a rear surface of the panel 1 for maintaining a vacuum state inside the cathode ray tube; a seal edge line 7a formed at a junction of the funnel 7 and the panel 1; a neck portion 8 formed behind the funnel 7; an electron gun 9 mounted to the neck portion 8 for emitting an electron beam; an inner shield 10 assembled to the frame 4 so as to shield the emitted electron beam from external magnetic fields; a deflection yoke 11 which surrounds an outer side of the funnel 7 for deflecting the electron beam; a reinforcing band 12 mounted at a skirt portion of the panel 1 for distributing stress and performing impact resistance; and a lug 13 for fixing the cathode ray tube. An axis of the cathode ray tube is labeled S.
Referring now to FIG. 2, the shadow mask 3 includes: an effective surface 3a on which circular or elliptical slots (not shown) are formed; and a skirt 3b having a constant height so as to be welded to the frame 4. The slots of the shadow mask 3 are arranged horizontally or vertically with a constant interval so that the electron beam can maintain a constant interval when the electron beam emitted from the electron gun passes through the slots and lands on the fluorescent film 1a. 
In operation, the electron gun 9 emits thermo electrons in accordance with inputted image signals. The emitted thermo electrons move forward towards the panel 1 by a voltage applied from each electrode of the electron gun 9 through acceleration and focusing processes. At this time, the thermo electrons are deflected by the deflection yoke 11 and pass through the slots formed on the shadow mask 3, thereby making color selection possible. Then, the thermo electrons collide with the fluorescent film 1a located at an inner surface of the panel 1 such that the thermo electrons excite the corresponding portion of the fluorescent film 1a, thereby displaying an image.
However, if the shadow mask is deformed by an external impact, some of the electron beams from the electron gun land on the wrong place on the fluorescent film deviating from an original intended position, thereby degrading color purity.
Hereinafter, impact resistance characteristics of the shadow mask for the external impact will be explained with reference to FIGS. 3 and 4. FIG. 3 is a side view showing a drop effect of the shadow mask illustrated in FIG. 2, and FIG. 4 is a graph showing a deformation mechanism of the shadow mask illustrated in FIG. 2.
As shown in FIG. 3, if an external impact is applied to the shadow mask 3, a drop effect occurs at a center portion of the shadow mask 3. Also, if the external impact exceeds the limitation point of the shadow mask 3, a plastic deformation is generated around the shadow mask. The external impact applied to the shadow mask is transmitted more greatly to the vertical direction of the curved surface of the shadow mask 3 rather than to the lateral directions thereof, which results in the plastic deformation of the shadow mask 3.
As shown in FIG. 4, if the external impact is applied to the curved surface of the shadow mask 3, the shadow mask undergoes a deformation for a constant time in proportion to the external impact. If the external impact exceeds the limitation point of the shadow mask 3, the deformed portion of the shadow mask can not be restored to the original state, thereby degrading color purity.
In order to solve these problems, there has been efforts to change the material and the thickness of the shadow mask, to form beads in the shadow mask, or to change the welding position of the skirt in the shadow mask. An amount of drop in relation to an external impact can be expressed by formula 1, (E * thickness T)/(M). As shown in formula 1, the amount of drop in the shadow mask is proportional to the Young's modulus E and the thickness T, and is inversely proportional to the mass M. A shadow mask formed of material with a high Young's modulus or with an increased thickness in accordance with the above principle has, however, increased the manufacturing cost of the shadow mask. Also, the effort to form embossment beams on the shadow mask has affected formation of the curved surface, without improving impact resistance of the shadow mask. In addition, the effort to form the welding point to fix the skirt and the frame of the shadow mask are fixed, near the curved surface of the shadow mask to reduce the effects of an external impact has caused the shadow mask and the frame to expand by the electron beam such that it has worsened the doming effect in which the electron beam is displaced from its originally intended position on the fluorescent surface, thereby degrading color purity.
There has also been an effort to design the panel, which is the basis for forming the shadow mask or its curvature, with a curvature that decreases gradually from the center portion thereof. However, limitations exist in controlling the thickness and the curvature due to the limitations of the panel fabrication. That is, a wedge ratio, which corresponds to thickness ratio between the center portion of a panel and the corner portion, of a color cathode ray tube having a flat outer surface center does not exceed 200%. For cathode ray tubes having a flatter outer surface center, the effect on impact resistance by gradually decreasing curvature radius reduces substantially, and if the curvature radius is significantly decreased, impact resistance deteriorates due to the increase of flat areas in the center portion of the cathode ray tube.